Actuators having a rotational output have use, for example, in heating, ventilation and air conditioning (“HVAC”) systems. One common use in HVAC systems is in a ventilation damper system, sometimes known as a variable air valve (VAV) damper system. In a VAV system, a ventilation damper (or set of dampers) can be rotated to various positions to control the amount of air flowing into or out of a space or shaft. A common VAV system configuration allows for the dampers to be opened from 0 degrees (fully closed) to 90 degrees (fully open).
One example of the use of a VAV system is to control cooling of a room in a building by regulating the flow of chilled air into the room. Another application of a VAV system is to regulate the amount of fresh air and recycled air that is circulated through the building HVAC system.
A significant concern in the design, construction and use of HVAC is operational efficiency. Because HVAC systems employ substantial energy resources, efficient operation advantageously can reduce the costs of operating a building. While inefficiencies can be identified in many aspects of an HVAC system, at least some are associated with VAV systems. For example, if a VAV system does not predictably or accurately open to an expected or desired amount, the VAV system may pass more or less air than expected. Such an error can be attributable to poor position control of the damper by the actuator.
As a result of poor position control of the damper, further control may be required to obtain a desired output, such as room temperature or fresh air content. Such further control requires additional energy for additional movement of mechanical structures, while initial error can result in efficient use of chilled, heated or fresh air.
In general, such losses can often be tolerated, even if they are not optimal. However, there are cases in which such inefficiencies and/or errors in actuator operation are particularly undesirable. For example, an actuator that controls the position of the sash of a fume hood system requires additional accuracy. In particular, because the fume hood is handling ventilation of noxious gasses, the “sash” of a fume hood needs to be controllable closed, and indeed typically requires accurate and complete closure under certain modes of operation.
Under such modes, inaccurate closing can result in overshoot or undershoot. Overshoot occurs when the actuator attempts to close the sash beyond 100% closed, and can result in damage to the sash or mechanical linkage members, among other things. Undershoot occurs when the actuator does not fully close the sash, and can result in leakage or improper handling of noxious or dangerous vapors. Overshoot and undershoot can occur from poor positional feedback and/or from poor control.
One way to avoid overshoot and undershoot, at least in cases of poor control is to dampen the step function response of the control algorithm. While further dampening reduces the possibility of overshoot, it comes at the cost of slower response time.
There is a need, therefore, for improved actuator operation to improve control in HVAC applications, including but not limited to those relating to fume hoods.